1. Field of the Invention
This invention relates to the structure of a composite overhead stranded conductor bundle in which optical fibers are inserted in an overhead electric wire or overhead ground wire.
2. Background Art
FIG. 1 is a section view showing the structure of a conventional composite overhead stranded conductor bundle. A plurality of optical fibers 1 are stranded around a tension member 2, and are then covered by a sheath 3, to form an optical fiber cable 4. The optical fiber cable 4 thus formed is inserted into an optical fiber protective tube 5, to provide an optical fiber unit 6. Aluminum-clad steel wires 7 are stranded around the optical fiber unit 6. Put another way, the optical unit 6 includes optical fiber cable 4 and a protective tube 5 housing therein the cable 4. The optical fiber cable 4 includes a tension member 2, a plurality of optical fibers 1 stranded around tension member 2, and a sheath 3 formed over the stranded optical fibers 1.
As is apparent from the above description, in the conventional composite overhead stranded conductor bundle, there is a gap between the optical fiber cable and the optical fiber protective tube, and therefore the optical fiber cable can be readily moved inside the protective tube. However, the movement of the optical fiber cable fluctuates because the friction between the optical fiber cable and the inner wall of the optical fiber protective tube is not uniform in the longitudinal direction. Furthermore, the movement fluctuates because of the bending of the composite overhead stranded conductor bundle. On the other hand, in general, a composite overhead stranded conductor bundle is exposed to quite severe conditions, such as temperature change, vibration and expansion, as compared with an ordinary communication cable. Accordingly, in the case when external stress such as expansion or contraction is applied to the conventional composite overhead stranded conductor bundle of FIG. 1, the fluctuation of the longitudinal friction between the optical fiber cable and the optical fiber protective tube variably affects the movement of the optical fiber cable in the longitudinal direction. As a result, stress is locally applied to the optical fiber cable as, for instance, when the optical fiber cable is slackened or bent, whereby the optical fiber transmission loss may be increased or the cable may be broken.